My old mouse was pretty loud so bought a new one, now it is too quiet. Any hacks to make it louder?
Buy a louder mouse. Adding anything underneath the mouse buttons would probably only increase the resistance of the buttons when trying to click, and may also prevent you from fully clicking. Additionally, if you care about warranties at all, opening up the mouse probably voids a warranty if there is one.
The good news is that mice are cheap.
You can use AutoHotKey to add a sound to every mouse click.
~LButton down::SoundPlay, E:\sounds\ldown.wav
Check out the Key List to find the names of the keys. I believe they are LButton, RButton, MButton, WheelUp, WheelDown.
The SoundPlay command can play a sound file for you. WAV should be the perfect format for this.
The ~ modifier in front of the button tells AHK to let the active window receive the keypress (or mouse button in this case) while still doing the AHK script. And the "down" modifier tells AHK to do this when the key is down. The counterpart to that is "up".